Depends a great deal on the amount of sun the box will get, but try beginning with vareigated ivy (Hedera helix 'Glacier'), which trails over the sides of the box and gives the box a light, shimmery touch. Then, add a dash of red with Calibrachoa 'Superbells Red', which should be placed near the front edge of the planter, allowing it to trail slightly over the rim and mix with the ivy. (Bonus: This easy-care variety, a relative of the petunia, is a great lure for hummingbirds.) Next, for small bursts of bright color, add yellow Osteospermum 'Symphony Lemon', an African daisy. While most African daisies don't bloom during the summer, the 'Symphony' series is the exception: its cheerful blooms stay strong through August and beyond. Finally, plant Verbena 'Superbena Blue-Purple' to pick up the violet color of the daisy centers. This plant, great for hot, sunny areas, is placed along the back of the planter and will soon grow to healthy proportions that easily fill out this portion of the window box...